Multiple myeloma continues to be associated with high morbidity and mortality rates due mostly to complications resulting from its effects on the skeleton. These complications reflect, in part, an impairment of osteoblast function and failure of bone formation. Currently available treatment modalities do not increase the already low bone mass in myeloma patients, and there is a compelling need for new treatment paradigms that enhance new bone formation to reverse the bone deficit. Although the mechanism underlying this inadequate osteoblast response in myeloma remains unknown, Dickkopf 1 (DKK1) has recently been implicated. Inhibition of the ubiquitin-proteasome (Ub-prot) pathway with small molecule inhibitors such as Velcade is having a significant impact on clinical management of myeloma patients with relapsed, refractory disease. We have recently made the exciting discovery that, in addition to their profound anti-tumor effects, proteasome inhibitors, including Velcade, also enhance bone formation in vitro and in rodents and data emerging from on-going clinical trials of Velcade in myeloma patients support these findings. We also find that Velcade is a potent inhibitor of DKK1 expression. Our hypothesis is that proteasome inhibition in myeloma affects both 'seed'(tumor) and 'soil', (bone microenvironment), and thereby has outstanding beneficial effects in the treatment of myeloma patients. This hypothesis is based on the fact that Velcade works through inhibition of the proteasome to block myeloma cell growth and also to restore osteoblast function by inhibiting DKK1 expression. Specifically, we propose that (i) both seed and soil are exquisitely sensitive to inhibition of the Ub-prot pathway;(ii) the precise mechanism in either case may be different but the E3 Ub ligase beta-TrCP appears to be centrally involved;(iii) Velcade has the potential to reduce tumor burden and concomitantly reverse bone disease, unlike other known chemotherapeutic approaches. Using the murine 5TGM1 model of myeloma bone disease as well as novel transgenic mouse models, coupled with novel state-of-the-art small animal imaging modalities that allow us to longitudinally track dynamics of tumor burden (microPET) and bone cell activity (microSPECT/CT), the proposed studies seek to elucidate the mechanisms mediating the beneficial effects of inhibiting the Ub-prot pathway in myeloma. The Specific Aims are to (I) Determine the effects of proteasome inhibition on Dkk1 and related bone disease of myeloma (ii) Determine the role of Dkk1 in bone formation in myeloma bone disease and the relationship to proteasome inhibition;(iii) Define the relationship between proteasome inhibition and beta-TrCP in cells within the tumor microenvironment (tumor cells and bone cells) in myeloma bone disease. The proposed studies will further our understanding of the mechanisms mediating the impact of proteasome inhibitors on tumor cells and the skeleton and suggest new molecular targets which potentially would serve as the basis for development of targeted therapeutics to treat myeloma bone disease.